This invention is related to the preparation of glasses suitable for sealing to molybdenum metal and is particularly directed to the production of glass envelopes for tungsten-halogen lamps wherein the glass is sealed to molybdenum metal electrical lead wires.
As the use of tungsten-halogen incandescent lamps has increased in the automotive industry, the manufacturers of glass tubing for the fabrication of envelopes for such lamps have expended considerable research resources to develop glasses suitable for that application, and which are also readily manufacturable. For example, envelopes for those lamps have been fashioned from fused quartz and 96% silica glasses. Those compositions exhibit strain points and thermal stabilities far in excess of the requirements for lamp envelopes. Unfortunately, however, those glasses are quite difficult to form and lampwork and, because of their very low coefficients of thermal expansion, when compared to molybdenum metal, they demand special techniques to seal the lead wires into the lamps.
Inasmuch as envelopes for tungsten-halogen lamps would be most economically produced from glass tubing, research has been directed to glass compositions demonstrating the thermal stability and viscosity parameters necessary for successful operation of the high speed Vello tube drawing process. Thus, the glasses must exhibit a viscosity at the liquidus of at least 50,000 poises, most preferably in excess of 60,000 poises, and a liquidus temperature below 1200.degree. C., most preferably below 1175.degree. C. The glasses must resist devitrification for relatively long periods of time at temperatures at or somewhat below their liquidus temperatures (thermal stability). Experience in lamp operation has demonstrated the need for the strain point of the glass to be in excess of 700.degree. C. and the linear coefficient of thermal expansion of the glass (0.degree.-300.degree. C.) to range between about 46-50.times.10.sup.-7 /.degree.C. Photoelastic examination of a cylindrical bead seal of the glass with molybdenum wire should indicate essentially zero tensile axial stress at room temperature, although up to about 150 ppm (parts per million) axial compressive stress can be tolerated. At the operating temperature of the lamp (.about.500.degree. C.), the cylindrical bead seal should indicate only slight tensile stress (no more than about 200 ppm).
Because alkaline earth aluminosilicate glasses can exhibit the physical properties required for envelopes used in tungsten-halogen lamps, much research has centered upon that composition field to formulate glasses suitable for the mass production of lamp envelopes, while concurrently manifesting the properties necessary for lamp operation. Such research is illustrated below in the Prior Art section of this application.